This invention relates to apparatus and a method for reducing the visibility of timing errors in for example, the inset image of a picture-in-a-picture (pix-in-pix) television display system.
In a pix-in-pix system, two images from possibly unrelated sources are displayed simultaneously as one image. The compound image includes a full size main image with an inset compressed auxiliary image. The subjective quality of the inset image may be affected by timing errors in either the main signal or the auxiliary signal.
Timing errors relevant to the present invention may occur, for example, when either the main or auxiliary signal is a nonstandard signal. As used herein, the term nonstandard signal means a video signal having a horizontal line period which may vary in length relative to the horizontal line period set by the signal standard to which the video signal nominally conforms (e.g. NTSC, PAL or SECAM). A noisy but otherwise standard signal may appear to be a nonstandard signal if the noise is of sufficient amplitude to mask transitions of the horizontal line synchronization (horizontal sync) signal.
To understand how these timing errors may affect the inset image, it is helpful to know how the auxiliary signal is processed and displayed. In a conventional pix-in-pix display system, the auxiliary signal is sampled at instants determined by a sampling clock signal which, desirably, bears a fixed relationship to the horizontal line scanning frequency of the auxiliary signal. To aid demodulation of the chrominance signal components of color television signals, the sampling clock signal desirably has a frequency that is a multiple of the chrominance subcarrier frequency. If the multiple is an even number, e.g., 4, for standard signals, this is a suitable sampling signal since, under all major video signal standards, it produces an integer number of samples per line interval. Under the NTSC system, this sampling clock signal may be developed, for example, by a phase locked loop which produces a sampling signal having a frequency of 4fc, four times the frequency, fc, of the color subcarrier signal, and which is locked in phase to color reference burst component of the auxiliary composite video signal.
The auxiliary video signal is separated into its component parts, generally a luminance signal and two color difference signals. These component signals are then subsampled both horizontally and vertically to develop signals that represent a compressed image. The lines of samples taken during one field of the auxiliary signal are stored in a memory. These samples are read from the memory for display using a clock signal that is desirably related to the horizontal line scanning frequency of the main video signal.
When the auxiliary signal originates from a noisy source or from a nonstandard source such as a video tape recorder (VTR) or a video game, the frequency of the horizontal sync signal may appear to vary significantly from line to line while the frequency of the color subcarrier signal, and thus of the color reference burst signal, may seem relatively stable. This variation can be caused by pickup head misalignment or by stretched tape in a VTR or by inaccuracies in the frequencies used by video game circuitry. Since, in the example set forth above, the sampling clock signal is locked in phase to the color reference burst signal, corresponding samples on successive lines may be shifted or skewed relative to each other. When these lines of samples are displayed in synchronism with the main signal, the pixels produced by these corresponding samples may not line up vertically. Consequently, any vertical lines in the inset image may appear jagged (if the period of the horizontal sync signal changes randomly) or tilted (if there is a fixed error in the relative frequencies of the horizontal sync and color burst signals). The frequency and phase variations which cause this type of image distortion are known as timing errors or, alternatively, as skew errors.
One type of timing error, which is relevant to the present invention, results from frequency or phase variations between the main horizontal sync signal and a video display clock signal that is phase locked to the color reference burst component of the main signal. Errors of this type may randomly change the distance between the left side edge of the main image (defined by the horizontal sync pulses) and the beginning of lines of the inset image (defined by the display clock signal). Main signal timing errors of integral numbers of sampling clock periods may be compensated for in the phase locked loop circuitry which generates the horizontal sync signal. Skew errors which are a fraction of a sampling clock period may be more difficult to correct.
One method of correcting these types of timing errors is to use interpolation to develop sample values that are matched to the clock signal used to store or display them. Another method is to shift the phase of the clock signal used to display the sample values so that it is properly aligned to the horizontal sync signal. These methods are described in U.S. Pat. No. 4,638,360 entitled "Timing Correction for a Picture-In-Picture Television System" which is hereby incorporated by reference.
Skew errors may also be corrected by generating samples that represent component video signals in synchronism with a skew shifted line locked clock signal. These samples are then applied to clock transfer circuitry which aligns the samples with a line-locked clock signal that is not skew shifted. U.S. Pat. No. 4,782,391 entitled "Multiple Input Digital Video Features Processor for TV Signals," which is hereby incorporated by reference, relates to a system of this type.
The first two methods described above use two substantially independent clock signals. Aside from the extra circuitry used to generate an additional clock signal, systems which use multiple clock signals may need to be carefully shielded to prevent radio-frequency interference between the signals.
In the third method described above, the luminance and color difference signal components of the auxiliary signal are separated by analog circuitry and then digitized. A system using this method may be more complex than a system which digitizes the composite video signal and then separates it into its component parts. In addition this second method uses line-locked clock signals, so it may be difficult to encode the color information signals of the compressed video signal so that the two signals may be time-division multiplexed for display.